The Institute of Electrical and Electronics Engineers (IEEE) has adopted a set of standards for wireless local area networks (LANs), known as 802.11. Wireless products satisfying 802.11a, 802.11b, and 802.11g are currently on the market, for example.
Recently, an 802.11n standard, known also as the Enhancement for High Throughput wireless standard, has emerged. Under the 802.11n standard, transmitters and receivers each have multiple antennas for transmission and reception of data. As a multiple input, multiple output (MIMO) technology, 802.11n is designed to coordinate multiple simultaneous radio signals, and is expected to support a bandwidth of greater than 100 megabits per second (Mbps).
In addition to supporting multiple antennas, the 802.11n standard defines a new channel width of 40 MHz, for higher transmission rates, where the previous wireless standards supported a 20 mega-Hertz (MHz) channel width. The 40 MHz channel occupies two neighboring 20 MHz channels, denoted as either primary or secondary. The channels are located either in a 2.4 Giga-Hertz (GHz) band or in 5.2 GHz band.
Under 802.11, a device (or client) accesses a wireless LAN (WLAN) by following a communication protocol, essentially involving the transmission of frames to an access point (AP). The frames may be data frames, control frames, or management frames. The frames convey information that enables the AP to “connect” the device to the WLAN.
Under 802.11n, devices supporting a 20 MHz channel width may coexist with devices supporting a 40 MHz channel width. The 802.11n specification allows a first 40 MHz-capable station to notify a second 40 MHz-capable station (or AP) to deliver all frames directed to the first station using a 20 MHz channel width. In some circumstances, the first station may also switch its own transmissions to a 20 MHz channel width.
A station operating in the 40 MHz mask mode may transmit and receive frames sent in the 20 MHz channel width. In this mode, the station cannot use pure 20 MHz filtering to support the 20 MHz mask, since the central frequency is still set for the 40 MHz mask. A station that transmits the 20 MHz channel width frame and does not use the 20 MHz mask is likely to produce much noise in the adjacent channel, as compared to a transmission using the 20 MHz mask. A station that receives frames sent in the 20 MHz channel, while this station operates in the 40 MHz mask, makes the receiving station more sensitive to noise in the adjacent channel, as compared to stations receiving in the 20 MHz mask.
Thus, in order for a 40 MHz-capable station to operate in a 20 MHz mode, the station needs to change the central frequency. The station needs to switch their central frequency when moving from 40 MHz mask to 20 MHz mask, and vice-versa. Such a switch may be needed, for example, where interference from another station causes noise to exist on the secondary channel.
The 802.11n specification defines AP administered switching between 40 MHz and 20 MHz channel width transmissions of the entire Basic Service Set that reserves time for the central frequency switching, known as phased coexistence operation (PCO). PCO is an optional feature, however, and thus does not solve the central frequency problem. Further, PCO does not allow any separate station switching between a 40 MHz mask and a 20 MHz mask on its own.
The 802.11n specification does not give the station that wants to change its mask enough time to switch the central frequency. If the station changes its central frequency when a frame intended for the station is transmitted, the station may lose the frame.
Thus, there is a need for a mechanism for a wireless LAN station to change the central frequency without losing a frame of data being transmitted to the station.